Moscow -LRB- CNN -RRB- -- The death toll in a Moscow metro derailment Tuesday has reached 22 , the state-run RIA Novosti news agency said .

Another 160 people are injured , of whom more than 120 were hospitalized , the Moscow health care department said . Of those , a few dozen were in critical condition .

The derailment took place during morning rush hour in the Russian capital . Three metro train cars derailed in a tunnel , the state-run ITAR-Tass news agency reported .

More than 200 people were evacuated , the Emergency Ministry said on its website .

Dozens of ambulances arrived at the scene , along with three helicopters , to help shuttle the injured to hospitals , ITAR-Tass said .

Moscow Mayor Sergei Sobyanin declared Wednesday as a day of mourning .

The cause is under investigation

It was not clear what caused the derailment , but the head of Moscow 's Transportation Department said it was not a terrorist attack .

However , it 's still subject to a criminal investigation as authorities seek to find out what 's behind the crash .

In the moments afterward , many of those caught up in the derailment scrambled back up to the surface at the station west of central Moscow , some bleeding and dazed as they emerged .

The rescue operation had become a recovery operation by Tuesday afternoon as emergency workers sought to free the remaining bodies from within the mangled train cars .

Their efforts were made more difficult by the depth of Moscow 's metro lines , on which roughly 11 million people travel daily .

The metro system -- the backbone of the city 's public transit network -- has been the target of past terror attacks .

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CNN 's Alla Eshchenko reported from Moscow , and CNN 's Laura Smith-Spark wrote from London . CNN 's Phil Black contributed to this report .

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NEW : The death toll rises to 22 , RIA Novosti says

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NEW : More than 120 people are hospitalized

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Three metro train cars derailed in a tunnel during rush hour , ITAR-Tass says

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The cause of the derailment is not clear ; officials have ruled out terrorism